1. Field of the Invention
This invention relates to secondary air supply system and secondary air supplying method of an internal combustion engine.
2. Description of the Related Art
An exhaust purifying catalyst, which is provided in an exhaust system of an internal combustion engine, has the function of reducing or removing harmful components contained in exhaust gas. The exhaust purifying catalyst normally exhibits its exhaust purifying capability in a specified temperature range. Thus, a secondary air supply system is known which causes new air to flow into exhaust ports when the temperature of the exhaust purifying catalyst is low, so as to re-burn harmful components contained in exhaust gas and expedite activation of the exhaust purifying catalyst, for reduction of the amount of the harmful components discharged or released to the atmosphere, as disclosed in, for example, Japanese Patent Application Publication No. 2004-124824 (JP-A-2004-124824) and Japanese Patent Application Publication No. 9-96214 (JP-A-9-96214).
However, if secondary air is supplied immediately after the engine is started, the harmful components, such as CO and HC, contained in the exhaust gas may not be re-burned because the temperature of the secondary air is low. In this case, the exhaust ports may be cooled by the low-temperature secondary air, which makes it difficult or impossible to activate the exhaust purifying catalyst, and the harmful components contained in the exhaust gas may be discharged into the atmosphere without being converted. To solve this problem, it is proposed in JP-A-2004-124824 to determine whether warm-up of the exhaust ports is completed by estimating the temperature of the exhaust ports, and start supply of secondary air when warm-up of the exhaust ports is completed, whereby the exhaust ports are prevented from being cooled by the secondary air.
However, neither JP-A-2004-124824 nor JP-A-9-96214 discloses a technology of estimating the exhaust gas temperature after the secondary air is supplied, by estimating the temperature of the secondary air itself.